iHandy Level Free
This was also one of my first apps and again, I rarely use it (but then the pictures at my house aren't exactly straight either). That said the fact that my phone has what is apparently called an accelerometer and thus can tell if something is level is just mind boggling.
I remember when cell phones were the size of bricks. Ooh, look, there's a gray hair! Seriously though, this app is pretty exciting and definitely useful should you want things level.